The present disclosure relates to a photogrammetric system and a photogrammetric method for generating a stereo model from images captured by a camera installed in a movable body.
Photogrammetry surveying techniques are known in which an image capture device such as a camera sequentially captures an object while moving relative to the object and the position of the device and the position of the object are measured from the captured images. Recent techniques use, for example, unmanned aerial vehicles (UAVs) as movable bodies. The UAVs are equipped with cameras and capture images from up in the sky.
Such a photogrammetry surveying technique is used to generate a stereo model based on feature points extracted from a plurality of captured images. Examples of the feature points include points characteristic of structures and landforms on the ground such as buildings, rivers, and mountaintops, and ground control points used as orientation points.
Conventional techniques are disclosed in, for example, Japanese Unexamined Patent Publication No. 2013-108927. In this disclosure, a flying object flies in meandering manner up in the sky over an object range to be measured and captures images of the object range. The flying object periodically captures images such that adjacent images in the advancing direction and adjacent images in adjacent courses overlap each other to a certain degree. After the flying object finishes capturing the entire object range, a single stereo image is prepared by relative orientation on one set of images adjacent to each other with regard to three images adjacent to each other in the advancing direction, and another stereo image is prepared by relative orientation on the other set of adjacent images. The two sets of stereo images are connected by using feature points extracted from a portion common to the two sets of stereo images, the portion being an overlapped portion of the three images. Subsequently, common tie points are selected from the images adjacent to each other in the adjacent courses and the adjacent stereo images in the adjacent courses are connected. With this process, a unified stereo image (stereo model) is prepared that covers the entire object range that has been measured and is represented by common three-dimensional coordinate system.
Conventional photogrammetry surveying techniques, such as Japanese Unexamined Patent Publication No. 2013-108927, typically use digital single-lens cameras that are commercially widely available. Such digital single-lens cameras typically include a focal-plane shutter that is a type of photographic shutter.
In the conventional photogrammetry surveying techniques, photogrammetric calculations are performed based on feature points on the assumption that all the objects in an image captured at a certain point of an area are captured simultaneously.
When an image is captured by using focal-plane shutter, portions of the imaging surface are exposed at slightly different timing in accordance with the travel time of the shutter curtains (light-excluding curtains). Exposure at different timing causes an image captured from a fast moving object such as a UAV to include errors (distortions) with regard to a relation between the positions in the images and the actual positions on the ground.
Such a problem occurs not only when physical shutter (mechanical shutter) such as focal-plane shutter is used, but also when electronic rolling shutter that linearly and sequentially exposes an imaging device from one side to the other side is used.
Since feature points include positional errors, performing feature-point-based photogrammetric calculations on the assumption that all the objects in an image are captured simultaneously results in reduction in accuracy in generating stereo model, which in turn reduces accuracy in photogrammetry.